Need to identify a virus

Hello.
The following happens on some friends' computer and I'm going along by using their descriptions.
It seems that as they type some text in Word or some Excel spreadsheet, there is some different text that gets generated and replaces whatever they write.
The text could be some silly sentence, asterisks, or random characters maybe. It happens randomly but once it does, it presumably makes it hard to type.
Their system is only protected by McAfee antivirus and they claim they run it but it found nothing.
I visited them, cleaned the computer with Ad-aware and SpyBot, and run TDS afterwards.
The keyboard was still hooked and the funny behaviour persisted.
They run WindowsXP and the virus seems to just be playful. It certainly doesn't try to hide.
Has somebody come across this and and can identify which one it might be?
Thanks in advance.

The extra scanning you did may not have found everything, virus-wise, so this will confirm it further. (The Panda online scanner is really quite good.)

However, the problem may not be a virus. It could be the built-in Windows XP voice recognition software. This is a common problem people experience, too. The solution, if that is it, is to disable it. See this article:

In a posting entitled "My Remote Keyboard is Possessed in XP," for example, one Microsoft customer reported "very strange behavior" that included letters appearing in input areas of the screen while browsing and writing e-mails.